1. Field of the Invention
This invention relates to a quenchable electronic flash unit having a safety circuit for eliminating any danger of electrical shock or injury to a flash unit user. More particularly, this invention relates to a safety circuit which employs a quench circuit of the flash unit to discharge a flash firing capacitor.
2. Description of the Prior Art
Electronic flash units typically employ a large capacitor having a capacitance between about 200 and 800 microfarads (uf), for storing electrical energy that is used for firing a flashtube. A DC to DC coverter circuit, which is energized from a low voltage battery (approximately 6 volts), charges the capacitor in a known manner to approximately 300 volts. A capacitor having a capacitance C charged to a voltage V stores electrical energy equal to 1/2 CV.sup.2. Thus, a 400 microfarad capacitor, charged to 300 volts, stores 18 joules of energy. If a flash unit user, in tampering with or trying to repair the unit, were to contact the terminals of a charged flash firing capacitor, discharge current from the capacitor could produce serious injury.
It is known for electronic flash units to include a safety circuit for discharging the firing capacitor to protect users against injury. For example, U.S. Pat. Nos. 3,666,964, 3,969,737 and 4,025,817 disclose electronic flash units having a circuit the sole purpose of which is to discharge a firing capacitor to prevent accidental injury to a user. These circuits generally include a normally open switch in series with a bleeder resistor, the switch and resistor being connected parallel to the firing capacitor. The switch is closed automatically whenever the housing of the flash unit is opened, thereby discharging the firing capacitor through the switch and the resistor. Although a circuit of this type has been found to operate reliably, both the switch and the bleeder resistor must be able to handle the high discharge current from the firing capacitor. Furthermore, both the resistor and switch are required solely as a safety feature, and therefore introduce additional complexity and cost into the flash unit.